The present invention pertains to a system utilizing circulating foam rubber balls as a cleaning media and, more particularly, to a ball straining mechanism for a circulating ball cleaning system used for cleaning the tubes in a heat exchanger.
It is well known in the art to provide the condenser in a heat exchanger with a large number of parallel tubes through which cooling water is directed. The condenser tubes are supplied with cooling water by a pump-operated recirculating system, typically receiving water from a cooling tower, circulating it through the heat exchanger condenser and returning it to the cooling tower.
Various methods are utilized for periodically cleaning the condenser tubes to remove deposits which accumulate therein. Typically, the condenser tubes may be about 1" in diameter and, in one known method, resilient foam rubber balls having a diameter slightly larger than the tubes are circulated therethrough with the cooling water. The balls are compressed slightly as they enter the tubes and are forced through the tubes by water pressure carrying accumulated deposits with them. The balls are injected into the cooling water flow from a parallel branch upstream from the condenser and are removed from the stream after they exit the condenser and diverted from the main cooling water flow back into the parallel branch for recirculation or collection. To separate the balls from the return flow to the cooling tower, a ball strainer comprising a large screen is disposed in the return flow piping system where the balls are screened from the flow and diverted into the collection/recirculation branch.
Ball strainers typically comprise an elongated screen section which is disposed at an angle in the main cooling water piping system. The downstream edge of the screen is elliptically shaped to closely fit the interior contour of the pipe when the screen is disposed at an acute angle to the pipe axis. Balls accumulating on the screen are funneled by the cooling water flow to the narrow downstream end where the apex of the elliptical screen meets the pipe wall. Here the balls accumulate and are diverted through a small opening in the wall of the pipe and directly into the small diameter collection/recirculation branch pipe. Thus, the primary flow of cooling water continues through the main piping system and a relatively small volume of cooling water is diverted with and carries the balls through the branch pipe, with the flow therethrough induced by a separate pump in the branch line.
Because of the shape and orientation of an elliptical ball strainer, it inherently creates a pinch point where its downstream apex meets the pipe wall. Special means are required to prevent balls from being trapped at the pinch point and accumulating such that they cannot be properly diverted and removed from the system. One prior art device comprises a collecting cup formed in the downstream end of the screen generally at the apex of the ellipse. The cup has a side wall which extends radially inwardly of the pipe to eliminate the pinch point at the ball diversion outlet. However, this construction requires the smooth planar elliptical surface of the screen to be interrupted and substantially complicates the manufacture of the screen and, therefore, the cost. Another prior art device utilizes an elliptical flange on the downstream portion of the screen, which flange is disposed generally perpendicular to the plane of the screen such that the edge of the lip engages the pipe to maintain the planar screen offset from the pipe wall. This also eliminates the sharp pinch point at the apex of the ellipse to prevent the jamming and accumulation of cleaning balls which might plug the ball diversion opening. Ball strainer screens are pivotally mounted inside the pipe such that they maybe rotated between a screening and a backwash position, the latter occurring after the balls have been completely diverted and temporarily collected in the branch line. However, the attachment of the large perpendicular lip to the downstream edge of the screen adds very substantially to the resistance to flow across the screen, making it difficult and requiring added power to rotate the screen between its backwash to its straining position. U.S. Pat. No. 4,385,660 is typical of prior art screening and diverting apparatus.